Free piston Stirling motors and heat pumps are exceptionally energy efficient and are durable mechanical devices for converting heat energy to mechanical energy or for converting mechanical energy to heat pumping work. Under some design conditions it is desirable to operate two or more free piston Stirling machines in a desired phase synchronism, that is so that the two engines run at a relatively constant phase angle between the periodic, approximately sinusoidal, motion of each of the two machines.
In the past multiple Stirling engines have been connected together in a ganged arrangement in which work spaces are interconnected so that the machine operates with a work space shared commonly by two different machines. The volume of the workspace is the sum of the volumes of each space plus the interconnection volume. Such machines operate with a 0 degree phase angle between the gas pressure waves of the different machines. This system provides adequate operation if the two machines are positioned relatively closely together. Close spacing is necessary because the interconnection between the two gas spaces is a part of the total work space volume seen by each of the two interconnected free piston Stirling engines. If the volume of the corrosion space becomes excessively large, the machines cannot operate effectively.
Similarly, the prior art has also directly connected together the displacer gas spring of each of two machines so that they act as a common gas spring in order to operate two free piston Stirling engines along an axis and cancel their vibrations. This arrangement, however, is also limited because the gas springs must be positioned very close together in the same housing so that the total gas spring volume is sufficiently small to provide the necessary spring constant needed to act upon the vibrating masses and operate the machine properly. The above system therefore required that the heat input ends of each of the two engines be at axially opposite ends of the coupled machines, which therefore required two separate heat energy inputs to the two separate spaced hot ends of the two different free piston Stirling machines.
Sometimes it is desirable, however, to utilize two separate free piston Stirling engines and to be able to synchronize their operation without requiring any particular portion of either machine to be physically closely near a physical portion of the other machine. For example, it is desirable to manufacture a single, free piston Stirling machine which can be operated by itself or alternatively may be connected with another identical machine and run in a desired synchronism with its operating characteristics being identical whether it is operated alone or in synchronism with another machine. This could not be done with prior art arrangements because connecting the machines in the conventional manner increases gas space volumes and therefore changes the operating characteristics.
It is also desirable in some design situations to align free piston Stirling machines along a common axis of reciprocation with the hot ends of each machine juxtaposed to receive heat from a single heat source. This or other reasons sometimes require that the gas spaces must be separated by such a substantial distance so that corresponding gas spaces cannot simply be interconnected and designed to operate as a single gas space.
There is therefore a need for an apparatus and method which permits two free piston Stirling machines to be operated in synchronism, but also permits each to be designed and operated with its own gas spaces in a manner such that connection of the machine to a second machine will not alter the design parameters or operation of either machine.
For example, such operation would be desirable when two, separate free piston Stirling engines are aligned in opposed orientation along a common axis of reciprocation. If such machines can be operated with their work space pressure waves in phase with each other, then their reciprocating parts will move in phase opposition, that is 180 degrees out of phase, so that the vibrations generated by each will be cancelled.